As a method for forming a fiber comprising a polymer material and having an outer diameter on a nanometer scale, an electrospinning method has conventionally been known. This method is a method in which a jet of a polymer solution charged with a high voltage is allowed to spurt toward the ground or an electrode plate (collector) having the opposite charge to the jet, whereupon the solvent volatilizes to deposit a fiber.
Generally, a polymer solution is extruded at a constant speed through a thin needle, such as a syringe needle, and when an electrostatic repulsive force caused on the surface of the polymer solution by the voltage applied to the solution exceeds the surface tension of the polymer solution, a jet of the polymer solution spurts from a droplet at the needle tip. From the thus produced polymer fiber, a thin film of a three-dimensional structure having a space network can be formed, and a function that has never been seen in a film can be imparted to the thin film.
By the way, in recent years, chitosan as a polymer material has being drawing attention. Chitosan is an N-deacetylation product of chitin, which is (1→4)-β-D-glucosamine having a linear structure, and which is represented by the following formula.

Chitosan is contained in shells of crustaceans, such as crabs and lobsters, and cell walls of mushrooms and the like, and biosynthesis of 100,000,000,000 tons (estimated) of chitosan occurs annually on the Earth simultaneously with biodegradation of chitosan, and thus chitosan is an abundant material of a recycling type. Utilizing excellent molecular functions of chitosan, chitosan is being put into practical use in various fields, e.g., the field of medical materials, the field of biotechnology, the field of foods, the field of agriculture, forestry, and fisheries, and the field of industry. For example, an attempt has been made to form a fiber of chitosan by the above-mentioned electrospinning method.
However, electrospinning of chitosan is generally difficult. The reason for this is that an acidic aqueous solution of high molecular-weight chitosan has high viscosity even when the concentration of the solution is low. The reason why the solution of chitosan has high viscosity even when the concentration of the solution is low resides in that chitosan inherently has a very high molecular weight (˜1,000,000), and further resides in that chitosan is a polymer electrolyte having an amino group in the molecular chain thereof. As shown in the formula below, an amino group (—NH2) is ionized (—NH3+) in water. Therefore, as shown in FIG. 12, electrostatic repulsion between the amino groups in the same molecular chain of chitosan causes the molecular chain to expand, so that the interaction between the adjacent molecular chains becomes stronger, increasing the viscosity of the solution.

For the above-mentioned reason, with respect to the balance between the solute concentration and the viscosity of a solution, there is a difference between a solution of a neutral polymer, which is not a polymer electrolyte, and a solution of a polymer electrolyte, such as chitosan, and this makes it difficult to electrospin the solution of a polymer electrolyte.
FIG. 13 shows a difference in the balance with respect to the low concentration and high concentration of a solution. From this, it is considered that the collective solution viscosity and the local interaction between branched chains with respect to the polymer electrolyte solution are totally different from those with respect to the neutral polymer solution.
There have been reported many methods for producing a fiber of chitosan by an electrospinning method, including non-patent documents 1 to 4. These methods can be roughly classified into the following three groups.
(1) A method in which a blend of chitosan and a synthetic polymer, such as polyethylene glycol or polyvinyl alcohol, is subjected to spinning.
In this method, the solution has a reduced chitosan content and the blended synthetic polymer makes up for the stringiness of the solution, enabling spinning of chitosan.
(2) A method in which electrospinning is conducted using a fluorine organic solvent (e.g., hexafluoro-2-propanol or trifluoroacetic acid).
In this method, a fluorine organic solvent having a lower viscosity and a lower surface tension as compared to an aqueous solvent is used to enable spinning of chitosan.
(3) A method in which a solution obtained by dissolving chitosan having a relatively low molecular weight (e.g., chitosan having a molecular weight of 100,000 or less) in concentrated acetic acid is subjected to electrospinning.
In this method, the use of low molecular-weight chitosan suppresses the increase of the viscosity of the solution due to entanglement of the molecular chains of chitosan, and the solution having a concentration as high as about 7 wt % makes up for the stringiness of the solution, and further the use of 90% concentrated acetic acid increases the charge density on the surface of the solution, thus enabling spinning of chitosan.
However, the methods of items (1) to (3) above have the following problems. Specifically, the chitosan nanofibers obtained by the method of item (1) above have a low chitosan content so that the properties of chitosan are not satisfactorily exhibited, and therefore the utilization of the obtained chitosan nanofibers in medical applications including the use as a wound bandaging material, environmental applications including the use as an ion-exchange filter, and the like is limited. The fluorine organic solvent used in the method of item (2) above is highly toxic to a human body and causes large loads on the environment, and further is expensive, and therefore industrial manufacturing of chitosan nanofibers using this method is difficult. The method of item (3) above is disadvantageous not only in that a further step for obtaining low molecular-weight chitosan is needed, but also in that the reduction of the molecular weight generally causes the physical properties of fibers to be poor.    Non-patent document 1: Morphological and Surface Properties of Electrospun Chitosan Nanofibers; Biomacromolecules 2008, 9, 1000-1006.    Non-patent document 2: Electrospinning of chitosan nanofibers: Degradation behavior and cellular response to normal human keratinocytes and fibroblasts; Biomaterials 2006, 27, 3934-3944.    Non-patent document 3: Electrospinning of chitosan dissolved in concentrated acetic acid solution; Biomaterials 2005, 26, 5427-5432.    Non-patent document 4: Electrospinning of Chitosan; Macromolecular Rapid Communications 2004, 25, 1600-1605.